1. Field of Invention
The present invention relates to a feedback circuit, and more particularly to a common-mode feedback circuit.
2. Description of Related Arts
The common-mode feedback circuit is usually used to stabilize the common-mode voltage of the fully differential analog circuit. The differential current which is generated by comparing the average of two differential output voltages with the common-mode reference voltage is fed back to adjust the common-mode voltage.
Referring to FIG. 1 of the drawings, an existing common-mode feedback circuit is illustrated, wherein the circuit comprises a power supply VDD, a ground GND, two input ends Vin+ and Vin−, two output ends Vout1 and Vout2, two resistors R, five field effect transistors M11, M12, M13, M14 and M15, and an operational amplifier.
The circuit detects an output common-mode voltage Vcm by the two resistors R, and compares the output common-mode voltage Vcm with an input reference voltage Vref by the operational amplifier. Vcm is forced to be equal to Vref. The operational amplifier outputs the feedback to the field effect transistor M13 for adjusting the current passing through the transistor M13 equal to the sum of the current passing through the transistor M11 and the current passing through the transistor M12, thus achieving the common-mode feedback.
However, the existing circuit which is capable of achieving the common-mode feedback occupies a large silicon area, which goes against integrating. Furthermore, the existing circuit increases the output load, thereby decreasing the gain.